The present invention relates to an electronic ballast-inverter circuit usable for powering fluorescent lamps or the like, and more particularly to an electronic ballast-inverter designed to reduce by half the high voltage stress on the inverter power switching transistors.
High frequency electronic ballast circuits (e.g. circuits operable at a frequency of at least 20 kHz) for powering fluorescent lamps generally provide the advantages of higher efficiencies and smaller size when compared with older 60 Hz ballast circuits. These high frequency ballast circuits, which usually include DC to AC inverters, have shortcomings in other areas, however, such as in the area of switching losses in the power switching transistors usually used in such applications, when operating at the power levels required by the fluorescent lamps. Such circuits have also generally been sensitive to the amplitude of the DC source voltage, in that a change in voltage will adversely affect the frequency of operation of such circuits. Third, these circuits generally have a very poor crest factor since they draw current from the AC supply in short bursts every half cycle of the input voltage, whereby the circuit (particularly if a large number of them are used) can adversely affect the input supply waveform. Fourth, it can be difficult to satisfactorily design these circuits to have a sufficiently high power factor. Fifth, where the inverter includes a pair of push-pull transistors, the base drive to each is usually through a resistor, an arrangement which creates an excessive amount of heat under certain conditions.
The reliability and efficiency of these circuits is also not good, in particular due to the need, in general, for an electrolytic capacitor at the input to the circuit for smoothing of the rectified AC sine wave power source. Although certain electronic ballast circuits have been designed so as to omit the need for an electrolytic capacitor, the use of the resultant unsmoothed or pulsed DC voltage has created the danger that both transistors in a push-pull inverter may go off at the same time, such that the voltage across each of the transistors will go up until breakdown occurs, thereby damaging the circuit. A circuit which overcomes many of the above described problems is described in applicant's copending U.S. patent application Ser. No. 204,561, now U.S. Pat. No. 4,388,562.
For circuits designed to operate at higher input DC source voltage levels, however, even this latter circuit may not be usable, due to the high voltage transients on the input power line which may be coupled across the transistor, where such transients are beyond the maximum voltage rating of the transistors. For example, when the on switch of a power supply is closed, the switch bounce inherent in this switch could create voltage transients exceeding 1,000-2,000 volts across the power transistor as a result of the inductance of the power line. Transistors rates to resist breakdown at the 2,000 volt level, for example, are not generally available at this time.
One method known in the prior art for reducing the adverse effect of power line voltage transients is to use a bridge inverter instead of the typical parallel transistor inverter. The bridge inverter provides the benefit that each transistor in the bridge inverter is subjected only to the voltage supply during cutoff because there is no induced voltage present derived from the operation of the power transformer. Thus, bridge inverters operate safely with twice the supply voltage for a given transistor type. A primary disadvantage of the bridge inverter, however, is its greater complexity, including the fact that it requires four transistors, instead of the two required in a normal parallel inverter, to provide the same power conversion. Half bridge converters are known in the art, but they have the disadvantage of being only able to generate a square wave output. Such an output creates harmonic distortion and inefficient power transfer to the fluorescent lamps. Half-bridge converters also do not provide for rapid turn off of the switching transistors, thereby ading to the inefficiencies inherent in such circuits.